Norwegian anti-immigration militant Anders Behring Breivik has spoken in open court for the first time and admitted killing 77 people in attacks in July, but he denied any guilt, saying he was a military commander in a far-right resistance movement.

Wearing a black suit, white shirt and silvery tie, a tense Breivik sat with his eyes mostly downcast and occasionally bit his lip in a packed hearing to extend his custody before trial.

At one point Breivik attempted to address survivors of Norway's biggest modern-day massacre, but the judge cut him off.

"I am a military commander in the Norwegian resistance movement and Knights Templar Norway," Breivik told the court.

It was the 32-year-old's first public utterance since he planted a car bomb on July 22 that killed eight people at an Oslo government building, then went on to shoot dead 69 more, most of them teenagers, at a Labour Party summer camp on the island of Utoeya.

"I acknowledge the acts, but I do not plead guilty," Breivik said, adding that he rejected the jurisdiction of the court because it "supports multiculturalism."

Oil-producing Norway, home to the Nobel Peace Prize, is known for its open society, peace and relative prosperity. The attacks sparked a public debate about immigration, security and a legal system which has never had to cope with such an event.

About 120 people packed into the courtroom. Hundreds more squeezed into overflow rooms equipped with video links. Towards the end of the hearing Breivik indicated with a finger that he wanted to speak again.

"I understand the aggrieved parties are present - may I say something to them?" he asked, but the judge turned him down and Breivik did not persist.

"HE AIMED AT ME"

Outside the courthouse protesters held a banner that read "No speaker's platform for fascists," echoing fears expressed by some victims and family members that Breivik would be permitted to expound his anti-immigration philosophy.

But after the hearing, a 20-year-old survivor of the island shooting said Breivik looked nervous and small, a far cry from the last time he saw the killer wearing a police uniform and carrying a semi-automatic rifle.

"The last time I saw him he actually aimed at me and fired," said Bjoern Ihler. "It was good today to see him reduced ... He fumbled a little and didn't catch anyone's eye. On the island he seemed very cold, calculated and precise in his movements."

Most of the island victims were in their teens or 20s. Some were shot at point blank range, others while trying to swim to safety.

Daniel Vister, another survivor, also said Breivik looked weak.

"I think that what he said there shows that he is completely mad," said Vister. "He is definitely not on this planet."

"He said he was tense coming in," defence lawyer Geir Lippestad said afterward. He said he did not know what Breivik had intended to say to surviving victims and family members.

In a rambling manifesto posted on the Internet before the attacks, Breivik wrote that his arrest would open "the propaganda phase" of his operation to ignite a war to defend Europe against a supposed Muslim takeover.

The hearing, required periodically under Norwegian law to keep a suspect imprisoned before trial, was Breivik's fourth but the first open to the public.

A district judge extended pre-trial custody for 12 weeks but said Breivik can begin receiving visitors and letters under strict control and that on December 12 he can have access to media for the first time, possibly even logging onto the Internet.

Breivik has been kept in solitary confinement since July 22 and has been denied visits, correspondence and access to newspapers and television.

Officials said court-appointed psychiatrists were expected to finish their work late this month and that a trial was tentatively set to begin on April 16.

They also said the courtroom used on Monday would be totally reconstructed for the trial, nearly doubling the audience capacity to 240 seats and adding a press centre for 250 to 300 journalists.